Since shortly after the discovery of X-rays, an X-ray intensifying screen, also called an X-ray conversion screen, has been used to convert X-ray energy to a more useful UV-visible light. The key constituent of an X-ray conversion screen is a phosphor material which absorbs incident X-ray photons and produces in their stead photons of UV visible energy. Such screens are now used widely in industry and medicine. In use, the screen, mounted in a cassette, is placed directly in the X-ray beam and comes into immediate contact with a sheet of photosensitive film which is more sensitive to the light emitted by the phosphor screen than to the X-rays. Thus, an "intensified" image is produced on the film.
Conventionally, in the fabrication of an X-ray conversion screen, the phosphor is made by mixing solutions or slurries of the individual ingredients or simply grinding the ingredients together, followed by a high temperature firing in various atmospheres (e.g., nitrogen, hydrogen, etc.) to achieve the desired result. The phosphor is then mixed with a suitable binder, coated on a support, and dried. An overcoat may also be applied to protect the product during use and to add to the usable life of the finished X-ray conversion screen.
While there are many known materials which luminesce, few have the special properties necessary to make them useful in X-ray intensifying screens. For example, the most widely used phosphor for X-ray screens for many years has been calcium tungstate and the screens made therefrom have been used as a standard by which other phosphors and screens are judged. In recent years, a number of other phosphors have been proposed for possible use in X-ray screens. For example, Brixner, U.S. Pat. No. 4,225,653 proposes the use of a number of blue- or green-emitting phosphors based on M' structure yttrium, lutetium and gadolinium tantalates. These tantalates may be further activated with rare earth materials (e.g., niobium, thulium, terbium, etc.) and mixtures of the phosphors may also be used. When the phosphors of Brixner contain niobium or thulium, the emission will be mainly in the blue while the use of terbium results in green emission. Although screens prepared using the phosphor of Brixner are noticeably faster and sharper than conventional CaWO.sub.4 screens, these new screens are noisier. The term "noise" in relationship to X-ray information theory applies to signals which do not carry useful information and the presence of which interferes with normal information transfer in the system. Noise is thus an objectionable phenomenon.
It is an object of this invention to provide a phosphor mixture suitable for making an X-ray intensifying screen with improved speed and sharpness and low noise.